Auroral Structures Research Articles

PFISR nightside observations of naturally enhanced ion acoustic lines, and their relation to boundary auroral features

Abstract. We present results from a coordinated camera and radar study of the auroral ionosphere conducted during March of 2006 from Poker Flat, Alaska. The campaign was conducted to coincide with engineering tests of the first quarter installation of the Poker Flat Incoherent Scatter Radar (PFISR). On 31 March 2006, a moderately intense auroral arc, (~10 kR at 557.7 nm), was located in the local magnetic zenith at Poker Flat. During this event the radar observed 7 distinct periods of abnormally large backscattered power from the F-region. These were only observed in the field-aligned radar beam, and radar spectra from these seven times show naturally enhanced ion-acoustic lines (NEIALs), the first observed with PFISR. These times corresponded to (a) when the polar cap boundary of the auroral oval passed through the magnetic zenith, and (b) when small-scale filamentary dark structures were visible in the magnetic zenith. The presence of both (a) and (b) was necessary for their occurrence. Soft electron precipitation occurs near the magnetic zenith during these same times. The electron density in the vicinity where NEIALs have been observed by previous studies is roughly between 5 and 30×1010 m−3. Broad-band extremely low frequency (BBELF) wave activity is observed in situ by satellites and sounding rockets to occur with similar morphology, during active auroral conditions, associated with the poleward edge of the aurora and soft electron precipitation. The observations presented here suggest further investigation of the idea that NEIALs and BBELF wave activity are differently-observed aspects of the same wave phenomenon. If a connection between NEIALs and BBELF can be established with more data, this could provide a link between in situ measurements of downward current regions (DCRs) and dynamic aurora, and ground-based observations of dark auroral structures and NEIALs. Identification of in situ processes, namely wave activity, in ground-based signatures could have many implications. One specific example of interest is identifying and following the temporal and spatial evolution of regions of potential ion outflow over large spatial and temporal scales using ground-based optical observations.

Modeling of the Jovian Auroral Environment and Its Effects on Spacecraft Charging

A well-known concern for polar orbiting spacecraft at the Earth is spacecraft charging due to the aurora. Studies of Jupiter reveal the presence of a variety of similar auroral phenomena. In particular, three regions have been identified-a narrow auroral zone at high latitudes, a complex and variable environment over the poles, and auroralike features associated with the main jovian moons and their magnetic flux tubes. These auroral structures are, like their earthly counterparts, expected to be sources of charging. This paper reviews the observational data and models of the jovian aurora zone and polar regions. In combination with models of the jovian plasma environment, the ambient charging currents are then computed with the intention of providing realistic estimates of spacecraft potentials at Jupiter. These are of particular importance to the Juno mission as it is a Jupiter polar orbiter and will pass through the jovian auroral zones near the planet. Of special concern, unlike previous missions to Jupiter, Juno will utilize large solar arrays. While previous missions (e.g., the Voyagers and Galileo) utilized RTGs and were carefully designed to avoid differential surface charging, the Juno mission's solar arrays may make it more sensitive to surface charging. Although the jovian aurora is a possibly serious threat to Juno and similar missions, as will be discussed, an understanding of the environment and proper mitigation techniques should limit their effects.

Initial observations of auroras by the multi-spectral auroral camera on board the Reimei satellite

Abstract The small Japanese scientific satellite Reimei was launched successfully from Baikonur Space Center into a sun-synchronous (noon-midnight meridian) polar orbit at an altitude of ∼630 km on August 23, 2005. Auroral fine structure observations have been performed by the multi-spectral auroral camera (MAC) and electron/ion spectrum analyzers (ESA/ISA) on board Reimei. The MAC is a three channel camera system used to simultaneously observe the temporal dynamics of the auroral fine structure at the wavelengths of 428 nm (N2 + 1NG), 558 nm (O), and 670 nm (N2 1PG). Since its successful launch, Reimei has been operated continuously and produced lots of unique data on the auroral fine structures with the MAC and ESA/ISA. The initial observation data from MAC has shown the temporal dynamics of auroral fine structure, such as the vortex street, appearing in the poleward edge of the auroral oval. The significant differences in the auroral shapes between the MAC channels suggest the existence of different emission processes. Further, the height profiles of the aurora and airglow are clearly seen in the image data in the limb direction. It is expected that the various mechanisms and dynamics of auroral fine structures can be investigated by analyzing the Reimei observation data.

PFISR observations of strong azimuthal flow bursts in the ionosphere and their relation to nightside aurora

Flow bursts within the ionosphere are the ionospheric signatures of flow bursts in the plasma sheet and have been associated with poleward boundary intensifications (PBIs). Some PBIs extend equatorward from the polar cap boundary, where they can be roughly divided into north–south-aligned and east–west-aligned structures. In this paper, we present two flow burst events observed by the new Poker Flat Advanced Modular Incoherent Scatter Radar (PFISR) in the pre-midnight auroral zone on 28 April 2007, one towards the west and the other towards the east. In both cases, enhanced flows lasted for about 8–10 min with peak velocities exceeding 1500 m/s. The concurrently measured electron density showed that the flow bursts occurred in low conductivity regions. However, near the poleward (equatorward) edge of the westward (eastward) flow burst, strong electron density enhancements were observed in the E region, indicating the presence of discrete auroral arcs. Auroral images from the Polar spacecraft were available at the time of the eastward flow burst and they indicate that this burst was associated with an east–west-aligned auroral structure that connected at later MLT to a north–south structure. In addition, simultaneous precipitating particle energy spectrum measured by the the Defense Meteorological Satellites Program (DMSP) F13 satellite reveals that this auroral structure resulted from mono-energetic electron precipitation associated with a significant field-aligned potential drop. These observations show direct evidence of the relationship between flow bursts, field-aligned currents and auroral intensifications, and suggest that eastward/westward flow bursts are associated with east–west-oriented PBI structures that have extended well within the plasma sheet. This is in contrast to the equatorward-directed flow that has been previously inferred for PBIs near the polar cap boundary and for north–south auroral structures. This paper illustrates the use of the PFISR radar for studying the magnetosphere–ionosphere coupling of flow bursts.

GPS scintillation in the high arctic associated with an auroral arc

A rapid signal‐fading event produced by diffractive scintillations was observed around 0123 UT on 8 November 2004 by three closely sited (less than 250 m apart) GPS scintillation receivers in northern Norway. The entire duration of the event was about 10 s and was recorded by all three receivers. Intense, short duration events such as these are not clearly observable in the 1‐min scintillation index (S4) because they do not necessarily last for the entire minute. In spite of their short duration they can cause a receiver to lose lock because of their intensity. The geomagnetic conditions were disturbed at this time with the interplanetary magnetic field southward for a period of several hours. Magnetometers from the IMAGE network in Scandinavia showed evidence of a 2000 nT substorm. The GPS measurements are compared with all‐sky camera (ASC) data to show that the signal fades can be attributed to the GPS ray paths crossing electron density structures associated with the aurora. The ASC images reveal moving auroral structures at the same time as the GPS signals show movement of the ionospheric regions causing fading. The results indicate that at high latitudes low‐elevation GPS signals can suffer sudden fading due to E‐region auroral events. This is the first time that a direct connection has been established between the loss of lock on a GPS receiver and diffractive fading caused by auroral precipitation.

Fine structure of auroras during auroral breakup according to the ground-based and satellite observations

The dynamics and fine structure of auroras before and during 60 auroral breakups, including pseudobreakups and breakups at moderate and high auroral activity, have been studied using the developed method for processing television images. The IMAGE and POLAR satellite and simultaneous ground images of auroras, ground magnetic data, and measurements of IMF and solar wind plasma parameters have been analyzed. The signatures that can be precursors of breakup have been found out in the auroral dynamics and morphology in the spatial—temporal vicinity of breakup. The morphological characteristics of auroral structures have been analyzed statistically. The directions of motion of weak subvisual structures have been determined. The velocities of motion of such structures are presented. The relation of the initial auroral arc bright-ening during breakups and pseudobreakups to the beginning of magnetic activation and formation of rayed structures has been analyzed.

Experimental tests of the eigenmode theory of auroral roar fine structure and its application to remote sensing

[1] Auroral roar is an electromagnetic emission near two and three times the electron gyrofrequency. The generation mechanism is believed to involve mode conversion of upper hybrid waves generated where the upper hybrid frequency matches a cyclotron harmonic and trapped in density enhancements. Theory predicts that if the density enhancement has dimensions comparable to the wavelength, the waves will be discretized, and indeed discrete features have been observed with the electric field detector on the HIBAR rocket experiment and with ground-level instruments. We have generated a database of eigenfrequencies predicted by theory as a function of the dimension and amplitude of the trapping density enhancement, assuming a Lorentzian-shaped cyclindrical trapping density enhancement embedded in the auroral F-region ionosphere. A fitting method is developed to test these predictions against the rocket and ground-based data. The fitting shows a significant discrepancy between theoretical predictions of density profiles and HIBAR observations. The predicted density enhancement scale size is of order ∼10 m, whereas rocket observations suggest scale size of order ∼100 m. A correction factor of 30 is used to reconcile the difference between the theoretical predictions and observation. A Versatile Electromagnetic Waveform (VIEW) receiver, operated at South Pole station during 2003, recorded many examples of auroral roar fine structures. Twenty-one relatively stationary examples of fine structures were selected, and the fitting procedure was applied to them, resulting in statistics of the trapping density enhancements associated with them, assuming that the eigenmode theory applies. In this case, the eigenmode theory indicates that irregularities in the range 7–40 m, or 200–1200 m if the correction factor is applied, would be responsible for generating observed auroral roar fine structures.

Scale sizes of intense auroral electric fields observed by Cluster

Abstract. The scale sizes of intense (>0.15 V/m, mapped to the ionosphere), high-altitude (4–7 RE geocentric distance) auroral electric fields (measured by the Cluster EFW instrument) have been determined in a statistical study. Monopolar and bipolar electric fields, and converging and diverging events, are separated. The relations between the scale size, the intensity and the potential variation are investigated. The electric field scale sizes are further compared with the scale sizes and widths of the associated field-aligned currents (FACs). The influence of, or relation between, other parameters (proton gyroradius, plasma density gradients, and geomagnetic activity), and the electric field scale sizes are considered. The median scale sizes of these auroral electric field structures are found to be similar to the median scale sizes of the associated FACs and the density gradients (all in the range 4.2–4.9 km) but not to the median proton gyroradius or the proton inertial scale length at these times and locations (22–30 km). (The scales are mapped to the ionospheric altitude for reference.) The electric field scale sizes during summer months and high geomagnetic activity (Kp>3) are typically 2–3 km, smaller than the typical 4–5 km scale sizes during winter months and low geomagnetic activity (Kp≤3), indicating a dependence on ionospheric conductivity.

Plasma sheet fine structure at high latitude

[1] High-resolution ion observations made in recent years, by the TIMAS instrument on the Polar satellite and other instruments, reveal a dynamic and finely structured plasma sheet, at least at high latitude. This study invokes multipoint Cluster observations with the CIS CODIF instruments (ion composition and distribution function) to determine whether transverse density gradients can be of the order of keV proton gyroradii scale size, as suggested by the TIMAS observations. It is shown that the plasma sheet is indeed prominently filamentary and that the proton density with 40 eV ≤ E ≤ 40 keV can vary by Δn = 0.4 cm−3 across less than five average proton gyroradii at R ≈ 5 RE (average E ≈ 7.5 keV at the time). This compares favorably with typical 10-km-size (or less) auroral structures when projected earthward.

Magnetospheric application of high-altitude long-duration balloon technology: Daylight auroral observations

Daylight auroral imaging is a proposed application of the NASA high-altitude long-duration balloon technology. This paper discusses the theoretical background of this application and test observations, for proof of the feasibility. It is demonstrated that nitrogen auroral emissions in the near-infrared band are detectable at altitudes of 35–40 km and above using a near-infrared InGaAs camera. The purpose of such observations is to identify auroral small-scale structures that are manifestations of auroral particle accelerations and the solar wind – magnetosphere – ionosphere interaction. Use of this new approach will enable studies of the dayside aurora, low-latitude aurora, and storm-time and substorm-time auroral conjugacy.

Multiple field line resonances: Optical, magnetic and absorption signatures

We present multi-instrument observations of ultra low frequency (ULF) wave activity from the dawn flank magnetosphere during the period 12:00–13:30 UT on the 16 December 2003. Optical, magnetic and riometer measurements from the Churchill line meridian in the Canadian sector are presented which demonstrate the presence of multiple discrete auroral arc structures accompanied by periodic magnetic and riometer absorption perturbations in the Pc5 (150–600 s) ULF band. Clear polewards propagation is demonstrated in all the instrument data sets, the magnetic signals showing most clearly the amplitude and phase characteristics consistent with discrete frequency field line resonances (FLRs) on closed field lines. Two discrete frequency field line resonant signals are apparent, at 1.8 and 3.0 mHz which resonate at approximately the same latitude. We explain this via the calculation of the Alfvén continuum, and show that both frequencies may be resonant in the same latitudinal region within instrumental resolution. The meridian scanning photometer (MSP) observations from polewards of the magnetometer determined resonant latitudes show evidence of low intensity (∼200 R) poleward moving discrete arcs related to the ULF waves. Interestingly the MSP observations demonstrate poleward phase propagation with variable rates across the field of view; faster apparent polewards phase propagation being seen at higher latitudes. We demonstrate that the complicated “braided” phase of the arcs can be explained via the precipitation resulting from the superposition of two discrete FLRs. Furthermore, we characterise the ≳25 keV energetic electron precipitation in the region of the FLRs and the arc structures via periodic D-region absorption. In this way, we link the magnetic and both soft and energetic particle precipitation signatures of FLRs together for the first time. Our results demonstrate that riometer absorption can be used to characterise FLRs, however, this is only generally possible at lower L-shells where energetic electrons in the ring current overlap with the FLR fields in the equatorial plane.

Meso-scale aurora within the expansion phase bulge

Abstract. We present ground-based optical, riometer and magnetometer recordings together with Polar UVI and GOES magnetic field observations of a substorm that occurred over Canada on 24 November 1997. This event involved a clear optical onset followed by poleward motion of the aurora as a signature of an expanding auroral bulge. During the expansion phase, there were three distinct types of meso-scale (10–1000 km) auroral structures embedded in the bulge: at first a series of equatorward moving auroral arcs, followed by a well-defined spiral pair, and finally north-south directed aurora (a streamer). The spirals occurred several minutes after the onset, and indicate a shear in the field-aligned current. The north-south aligned aurora that formed about 10 min after the onset suggest bursty bulk flow type flows taking place in the central plasma sheet. Polar UVI observations of the polar cap location indicate that the southward drifting arcs were associated with magnetospheric activity within closed field lines, while the auroral streamer was launched by the bulge reaching the polar cap boundary, i.e. the mid-tail reconnection starting on the open field lines. The riometer data imply high energy electron precipitation in the vicinity of the the poleward moving edge of the auroral bulge, starting at the onset and continuing until the formation of the north-south structure. In this paper, we examine this evolving auroral morphology within the context of substorm theories.

Fast Auroral Snapshot observations of perpendicular DC electric field structures in downward auroral current regions: Morphology

We present an analysis of Fast Auroral Snapshot perpendicular electric field observations using both ion distributions and electric field measurements in auroral downward current regions to study the full DC E⊥ (electric field perpendicular to the geomagnetic field) and potential structures. We use the electric field data from the field instrument for the spin‐plane component of E⊥, and the ion drift measurements for the axial DC E⊥. Examining 71 return current region crossings, a significant fraction (more than half) show E⊥ signatures during strong field events indicative of curved potential structures rather than idealized straight arcs. We define sheetlike structured events as those for which the ratio of the two E⊥ components remains constant during the spacecraft crossing, and curved structures as those where the ratio varies. Sheetlike structures can be interpreted as straight arcs, but curved structures require gradients in another dimension. A statistical comparison shows parametric differences between sheetlike and curved structures; however, the distinction between sheetlike and curved events cannot be sorted clearly using j∥, E⊥, or scale length. Using the full DC electric field vector, we can investigate the morphology of auroral return current potential structures and inconsistencies with idealized static return current models. In this paper we present the observed spatial structures; in a companion paper we explore the implications of these observations.

Auroral poleward boundary intensifications (PBIs): Their two‐dimensional structure and associated dynamics in the plasma sheet

Auroral poleward boundary intensifications (PBIs) are typically seen both in ground meridian scanning photometers (MSP) and in ground and spacecraft auroral images. They appear as a localized increase in intensity at or near the magnetic separatrix. This increase is often seen to extend equatorward through the ionospheric mapping of the plasma sheet. PBIs are associated with plasma sheet flow bursts and are thus important for the remote monitoring of plasma sheet dynamics. From the study of simultaneous ground MSP observations, IMAGE FUV auroral images, and Geotail plasma sheet data, we find that PBIs correlate well with plasma sheet fast flows observed within the local time sector of the PBIs and that there can be several PBIs over the longitudinal range of fast flows in the tail. We infer that every north‐south PBI is the ionospheric signature of a fast flow channel in the plasma sheet and that many fast flow channels exist simultaneously over a width of the plasma sheet that can comprise the whole width of the plasma sheet or only a part of it. Also, we find that there is a local time dependence on the type of PBI structure. Most PBIs are narrow auroral structures that are not strictly north‐south oriented. Instead, PBIs are tilted counterclockwise away from the north‐south direction, leading to a preferred orientation that is approximately aligned with a line going from 0300 magnetic local time (MLT) to 1700 MLT. This results in PBIs that are closer to north‐south (NS) structures in the postmidnight sector and closer to east‐west (EW) near the dusk sector. In the premidnight sector (2200–0000 MLT), PBIs start as EW arcs and then tilt and become primarily NS structures. We further found for one event that the PBI fast flows have a large Vy component resulting in tail convection that is both earthward and dawnward in the region of Geotail. We suggest that the continuous and strongly positive interplanetary magnetic field (IMF) By may order the two‐dimensional convection as observed, thus offering a possible explanation for the alignment direction of PBIs in the ionosphere under the assumption that fast flow channels align themselves with the background convection. However, the projection of the PBI structures into the tail using the T96 model suggests that all PBIs, both EW and NS, map to radially stretched channels in the tail that do not have a significant dawnward component. Future work is needed to clarify this apparent contradiction. Finally, frequency analysis indicates that the PBI/bursty bulk flow (BBF) period is characterized by oscillations in the velocity and magnetic field with frequencies of ∼0.6 mHz and ∼1.3–1.5 mHz. This oscillation in velocity is superposed on the background strong convection.

Substorm dynamics revealed by ground observations of two-dimensional auroral structures on 9 October 2000

Abstract. Multi-instrument observations of a small substorm event on 9 October 2000 provide, with high time resolution, two-dimensional information about substorm dynamics. A sequence of three optical intensifications, each associated with a Pi2 burst, were found, in which the third auroral intensification marked the onset of a small substorm. All three intensifications originated close to midnight, but evolved progressively eastward. Within each of the three optical intensifications, a few azimuthally-spaced patches appeared, the first one near midnight and the subsequent patches successively eastward in the postmidnight sector, providing evidence for drift wave activity in the near-geosynchronous-orbit plasma sheet (NGOPS). The SuperDARN measurements reveal the development of eastward electric fields at NGOPS latitude within each Pi2 burst interval. These observations can be interpreted as supporting the drift-Alfvén-ballooning (DAB) mode instability and its role in substorm expansion at NGOPS.

Temporal and spatial evolution of discrete auroral arcs as seen by Cluster

Abstract. Two event studies are presented in this paper where intense convergent electric fields, with mapped intensities up to 1350 mV/m, are measured in the auroral upward current region by the Cluster spacecraft, at altitudes between 3 and 5 Earth radii. Both events are from May 2003, Southern Hemisphere, with equatorward crossings by the Cluster spacecraft of the pre-midnight auroral oval. Event 1 occurs during the end of the recovery phase of a strong substorm. A system of auroral arcs associated with convergent electric field structures, with a maximum perpendicular potential drop of about ~10 kV, and upflowing field-aligned currents with densities of 3 µA/m2 (mapped to the ionosphere), was detected at the boundary between the Plasma Sheet Boundary Layer (PSBL) and the Plasma Sheet (PS). The auroral arc structures evolve in shape and in magnitude on a timescale of tens of minutes, merging, broadening and intensifying, until finally fading away after about 50 min. Throughout this time, both the PS region and the auroral arc structure in its poleward part remain relatively fixed in space, reflecting the rather quiet auroral conditions during the end of the substorm. The auroral upward acceleration region is shown for this event to extend beyond 3.9 Earth radii altitude. Event 2 occurs during a more active period associated with the expansion phase of a moderate substorm. Images from the Defense Meteorological Satellite Program (DMSP) F13 spacecraft show that the Cluster spacecraft crossed the horn region of a surge-type aurora. Conjugated with the Cluster spacecraft crossing above the surge horn, the South Pole All Sky Imager recorded the motion and the temporal evolution of an east-west aligned auroral arc, 30 to 50 km wide. Intense electric field variations are measured by the Cluster spacecraft when crossing above the auroral arc structure, collocated with the density gradient at the PS poleward boundary, and coupled to intense upflowing field-aligned currents with mapped densities of up to 20 µA/m2. The surge horn consists of multiple arc structures which later merge into one structure and intensify at the PS poleward boundary. The surge horn and the associated PS region moved poleward with a velocity at the ionospheric level of 0.5 km/s, following the large-scale poleward expansion of the auroral oval associated with the substorm expansion phase. Keywords. Ionosphere (Ionosphere-magnetosphere interacctions; Electric fields and currents; Particle acceleration)

Ion acoustic auroral structures in the presence of hot ion precipitation in the upper ionosphere

The linear and nonlinear ion acoustic waves with electric field parallel to the external magnetic field have been investigated in a two‐ion component plasma. The theoretical model is applied to the upper ionosphere to explain the solitary structures with predominantly parallel polarization and density fluctuations observed by Freja. The hot ion precipitation to the ionospheric region has been taken into account. The estimates made by the present theoretical model for electric field magnitude (∼100 mV/m) and density humps (15–20)% within the nonlinear solitary structures are in good agreement with the observations. The vertical scale size of the structure is predicted to be of the order of a few meters depending upon the background density and electron temperature.

Dynamical behavior of U-shaped double layers: cavity formation and filamentary structures

Abstract. Observations from the Polar and FAST satellites have revealed a host of intriguing features of the auroral accelerations processes in the upward current region (UCR). These features include: (i) large-amplitude parallel ( ) and perpendicular () fluctuating as well as quasi-static electric fields in density cavities, (ii) fairly large-amplitude unipolar parallel electric fields like in a strong double layer (DL), (iii) variety of wave modes, (iv) counter-streaming of upward going ion beams and downward accelerated electrons, (v) horizontally corrugated bottom region of the potential structures (PS), in which electron and ion accelerations occur, (vi) filamentary ion beams in the corrugated PS, and (vii) both upward and downward moving narrow regions of parallel electric fields, inferred from the frequency drifts of the auroral kilometric radiations. Numerical simulations of U-shaped potential structures reveal that such observed features of the UCR are integral parts of dynamically evolving auroral U-shaped potential structures. Using a 2.5-D particle-in-cell (PIC) code we simulate a U-shaped broad potentialstructure (USBPS). The dynamical behavior revealed by the simulation includes: (i) recurring redistribution of the parallel potential drop (PPD) in the PS, (ii) its up and downward motion, (iii) formation of filaments in the potential and density structures, and (iv) creation of filamentary as well as broad extended density cavities. The formation of the filamentary structures is initiated by an ion-beam driven instability of an oblique ion mode trapped inside a broad cavity, when it becomes sufficiently thin in height. The filaments of the PS create filamentary electron beams, which generate waves at frequencies above the lower hybrid frequency, affecting plasma heating. This results in plasma evacuation and formation of a cavity extended in height. The waves associated with filamentary electron beams also evolve into electron holes. The transverse and parallel scale lengths of the regions with large and as well as their magnitudes are compared with satellite data.

Variations of auroral hydrogen emission near substorm onset

Abstract. The results of coordinated optical ground-based observations of the auroral substorm on 26 March 2004 in the Kola Peninsula are described. Imaging spectrograph data with high spectral and temporal resolution recorded the Doppler profile of the Hα hydrogen emission; this allows us to estimate the average energy of precipitating protons and the emission intensity of the hydrogen Balmer line. Two different populations of precipitating protons were observed during an auroral substorm. The first of these is associated with a diffuse hydrogen emission that is usually observed in the evening sector of the auroral oval and located equatorward of the discrete electron arcs associated with substorm onset. The average energy of the protons during this precipitation was ~20–35 keV, and the energy flux was ~3x10–4Joule/m2s. The second proton population was observed 1–2min after the breakup during 4–5min of the expansion phase of substorm into the zone of bright, discrete auroral structures (N-S arcs). The average energy of the protons in this population was ~60 keV, and the energy flux was ~2.2x10–3Joule/m2s. The observed spatial structure of hydrogen emission is additional evidence of the higher energy of precipitated protons in the second population, relative to the protons in the diffuse aurora. We believe that the most probable mechanism of precipitation of the second population protons was pitch-angle scattering of particles due to non-adiabatic motion in the region of local dipolarization near the equatorial plane.Keywords. Auroral ionosphere; Particle precipitation; Storms and substorms

Variable morphology of Saturn's southern ultraviolet aurora

The Space Telescope Imaging Spectrograph camera on board Hubble Space Telescope obtained 68 FUV images of Saturn's southern auroral emission between 8 and 30 January 2004, during Cassini's approach to Saturn's magnetosphere. The HST observations took place in four different solar wind regimes with a low‐field rarefaction region from 8 to 16 January, a minor compression event on 17 January, a rarefaction region with intermediate field strengths from 19 to 25 January, and a major compression region from 26 to 30 January. The images have been projected onto polar maps in order to characterize and compare the general morphology of the auroral emission. The first 20 images were obtained during a period covering about 70% of one full rotation of Saturn. They show that the bright ring of auroral emission actually consists of several arcs of different width and brightness and forming along different parallels. Overall, the auroral region is shown to rotate at ∼65% of the full planetary rotation, although the angular velocity of some isolated auroral structures constantly decrease with time, down to 20%. The strongest auroral precipitations are observed in the morning sector. The polar projections of the 48 remaining images confirm dramatic changes in morphology, characterized by different average zones. During the campaign, short intervals during which the auroral region is significantly contracted and clearly forms a spiral were followed by intervals of reinflation of the auroral region. It is suggested that the two major auroral contraction events corresponded to the arrival of the solar wind shocks observed by Cassini on 17 January and 25 January. The present analysis indicates that Saturn's auroral morphology responds to the solar wind conditions at Saturn.